The choice of the red cell system for aging studies is based on the following considerations: (1) these cells have a relatively short but definite life-span (120 days) which is apparently predetermined and not the result of random destruction; (2) cells of different age can be easily separated by difference in density, thus avoiding complications involved in working with cultured cells; (3) the cell membrane is relatively simple, allowing more clear-cut information in molecular terms by obtained. The spectrin and actin network on the cytoplasmic side of the red cell membrane is believed to be an important determinant of cell deformability and osmotic fragility. We plan to use both biochemical and biophysical approaches to elucidate the role of these contractile proteins in cell aging, because of the well known phenomena of decreased deformability and increased susceptibility to osmotic lysis in old cells. The biochemical approach will be on the characterization of the contractile proteins in the intact cell and in isolated form, including studies on the degree of phosphorylation, state of aggregation or cross-linking, sensitivity to various agents (e.g. cytochalasins, local anesthetics, ionophores, and prostaglandins), and binding of (3H) cytochalasin B and (3H) dihydrocytochalasin B. The biophysical approach involves the measurement of the fluidity of the lipid region of the membrane, using electron paramagnetic resonance techniques, in cells of differen age and under different experimental conditions. In addition, the physical properties of spectrin and action isolated from young and old cells will be studied with viscometry and light scattering techniques. With these studies, we hope to establish a correlation between the state of the contractile proteins and the age-related rigidity of the membrane.